This just in, history is complicated, there were also plenty of tribes that sided with the Union and did not practice slavery. Native Americans are not a monolith, we’ve known that since like, the Seven Years War, where some tribes were on the French side and some were on the British side.
A lot of native tribes did practice slavery; some also aligned with the CSA because they figured they had a better shot of protecting their land if the CSA won the war. A lot of tribes stuck with the union, however, or stayed neutral — always important to remember that Native Americans are not a monolith and belong to distinctly different cultures.
I mean, in pretty much every war that happened on American soil prior to the Civil War, there were Natives on both sides. In the Revolution, there were patriot natives and loyalist natives. In 1812 there were US-aligned natives and British-aligned natives, actually one of the biggest knocks by Cherokee people against Andrew Jackson is “we fought under Andrew Jackson’s command in 1812, and he turned around and put us on the Trail of Tears.” And yeah there were Confederate natives.
And a lot of the Confederate-aligned natives cited the Union’s treatment of natives, specifically the Indian Removal Act, as a reason they joined the Confederate effort. Obviously that was a stupid decision with hindsight, but it is undeniable that the Union wasn’t any better to natives than the Confederacy was.
yeah some Native American groups practiced slavery even before European colonization - but important to specify it wasn’t exactly like chattel slavery, where you and all your descendants are treated as property for hard labor. people could (sometimes) be enslaved temporarily or be adopted as a full member of the tribe. post-colonization, many native ppl were enslaved alongside black ppl. some “civilized” tribes tried assimilating into white culture, including via chattel slavery.
Usually at the worst pre-colonization, you could be enslaved for life, but your children would be born members of the tribe that you were enslaved by. And yeah, Europeans used Natives for slave labor especially early on, but those Natives kept dying of smallpox, which is when they started the Trans-Atlantic slave trade.